1. Field of the Invention
The invention relates to a failure diagnostic technology, and more particularly to a failure diagnostic apparatus for an intake air stream control device provided in an intake system for the internal combustion engine.
2. Description of the Related Art
As an example of such an internal combustion engine, an internal combustion engine is known, which includes an intake air stream control device such as a swirl control valve (hereinafter, referred to as “SCV”), and a jet flow port. Also, an internal combustion engine is known, in which a swirl or a tumble flow suitable for a present operating state is formed in a combustion chamber by operating the intake air stream control device, whereby lean burn operation that is known can be performed.
More specifically, fuel in appropriate concentration is diffused to appropriate portions of the combustion chamber by forming the air stream in the combustion chamber, and this diffusion of the fuel makes a combustion state stable while only a small amount of the fuel is used.
In relation to the intake air stream control device, for example, Japanese Patent Application Publication No. JP-A-9-42021, Japanese Patent Application Publication No. JP-A-7-83101, and Japanese Patent Application Publication No. JP-A-2001-20782 disclose failure diagnostic technologies for the SCV.
According to the failure diagnostic technology disclosed in the Japanese Patent Application Publication No. JP-A-9-42021, a failure of the SCV is detected using pressure in a cylinder (hereinafter, referred to as “cylinder pressure”) and an output of an air-fuel ratio sensor.
More specifically, a degree of stability of the combustion in the engine is monitored by detecting the cylinder pressure using a pressure sensor when the SCV is closed. In addition, under this situation, an air-fuel ratio is controlled to become the leanest value at which the degree of stability of the combustion in the engine can be maintained, and the leanest value is regarded as a lean limit value. When the lean limit value is leaner than a reference air-fuel ratio, it is determined that the SCV is not operated normally.
Also, according to the failure diagnostic technology disclosed in the Japanese Patent Application Publication No. JP-A-7-83101, a combustion period from when the ignition is started until when the cylinder pressure reaches a peak value is detected based on a signal of a crank angle sensor when the SCV is in a closed state or in an opened state during normal operation; an actual combustion period is detected based on a signal of a cylinder pressure sensor; and it is determined whether a failure has occurred in the SCV by comparing the combustion period detected based on the signal of the crank angle sensor to the actual combustion period.
Also, according to the failure diagnostic technology disclosed in the Japanese Patent Application Publication No. JP-A-2001-20782, a combustion state when the SCV is in the closed state and a combustion state when the SCV is in the opened state are detected, for example, based on an output value of a knock sensor; and when it is determined that the combustion state deteriorates as a result of comparison between both of the combustion states, it is determined that a failure has occurred in the SCV.
In the conventional failure diagnostic technologies, a change in the combustion state which depends on the position of a valve element of the intake air stream control device is detected by the cylinder pressure sensor or the like, and the failure diagnosis for the intake air stream control device is performed by comparing a normal change in the combustion state to the detected change in the combustion state.
Therefore, for example, when the valve element incorporated in the intake air stream control device falls off the intake air stream control device, or when the intake air stream control device remains in the opened state, it is not possible to accurately detect the change in the combustion state when the intake air stream control valve is in the closed state. Accordingly, it is necessary to additionally provide an opening degree sensor for directly detecting the position of the valve element of the intake air stream control device.
As described above, in the failure diagnostic technologies disclosed in the Japanese Patent Application Publication No. JP-A-9-42021, and the Japanese Patent Application Publication No. JP-A-7-83101, the failure diagnosis is performed by detecting the combustion state based on the output value of the cylinder pressure sensor. The cylinder pressure sensor is more expensive than other sensors, and it is necessary to perform extensive machining operation for a cylinder head when the cylinder pressure sensor is installed. Thus, it costs much, and it is troublesome to employ the conventional failure diagnostic technologies for an existing internal combustion engine. Accordingly, the conventional failure diagnostic technologies need to be further improved.